A lithium manganese oxide is used as a cathode active material for a lithium ion secondary battery. As its manganese material, electrolytic manganese dioxide is widely used. Further, trimanganese tetraoxide is also used as a preferred manganese material of a lithium manganese oxide, whereby the fillability of the obtained lithium manganese oxide tends to improve (for example, Patent Documents 1 and 2).
Trimanganese tetraoxide is converted to a lithium manganese oxide by being mixed with a lithium material and another metal material, followed by firing. For example, rhombic LiMnO2 obtained by mixing and grinding lithium hydroxide and trimanganese tetraoxide, followed by firing, has been reported (Patent Document 1). Further, a lithium nickel manganese cobalt oxide obtained by forming lithium carbonate, trimanganese tetraoxide, cobalt oxyhydroxide, nickel hydroxide and the like into a slurry, and wet grinding the slurry, followed by firing, has been reported (Patent Document 2).
As mentioned above, trimanganese tetraoxide to be a manganese material of a lithium manganese oxide, which is used as mixed with another material, is required to have high mixing properties with another material, that is, to be uniformly mixed with another material.
As trimanganese tetraoxide to be a manganese material of a lithium manganese oxide, for the purpose of uniform mixing, trimanganese tetraoxide having a maximum particle size of at most 150 nm, obtained by mixing a manganese-containing liquid with an alkali liquid, followed by oxidation, has been reported (Patent Document 3). Further, trimanganese tetraoxide having a large average primary particle size of from 3.0 to 15 μm and having a low Na content and a low S content, in order to obtain lithium manganate particle powder having a large primary particle size and having a small amount of agglomerated particles, has been reported (Patent Document 4).